It's not the behaviour that would be a problem for me, it's telling if they are all playing the same note without looking at their fingers! And if you can't give them feedback, it's not such a good lesson- drumming would be easier for me as no "notes" to hear ( have an adequate sense of rhythm).
I teach whole class recorder, ukulele and drumming (using buckets!) as PPA cover to my Year 3 classes. I usually have a TA with me, but not always. It's completely doable for a teacher who has good behaviour management skills. I do a term of each finishing with a short concert to parents so it gives pupils a taste of playing different instruments before they choose peri instruments for Year 4. It can be a bit headache inducing!
Well, a caveat here that I am not musical and don't have a good ear, but I teach recorder clubs and struggle with more than six, the music teacher has asked me to do 10 and I find it v hard- I can't tell who has it right! And I am an experienced teacher ( ofsted outstanding!) who can teach most stuff to 30 kids! I would be pretty challenged if asked to teach my whole class the same instrument all at once. In fact I couldn't stand it!